🔧 Highlander - Spark Plug Replacement

Replacing the spark plugs on your Highlander helps restore smooth starting, clean idle, fuel economy, and proper ignition performance. The 3.5L V6 uses six spark plugs: three are easy to reach at the front bank, and three require removing the upper intake manifold to access the rear bank.

Difficulty Level: Intermediate | Estimated Time: 3-5 hours

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⚠️ Safety & Precautions

• ⚠️ Work only on a completely cool engine to avoid burns and thread damage in the aluminum cylinder heads.
• ⚠️ Disconnect the negative battery cable before unplugging ignition coils and intake wiring.
• ⚠️ Keep dirt, bolts, and tools out of the intake ports. An intake port is an opening that leads directly into the engine.
• ⚠️ Do not use anti-seize on modern Toyota iridium spark plugs unless the plug manufacturer specifically requires it.
• ⚠️ Use a torque wrench. A torque wrench tightens bolts to the correct force so parts are not damaged.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm socket
• 5/8-inch spark plug socket
• 3/8-inch drive ratchet
• 3/8-inch drive torque wrench
• 6-inch extension
• 3-inch extension
• Universal joint socket adapter
• Flathead screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Magnetic pickup tool
• Compressed air nozzle
• Shop vacuum
• Clean shop towels
• Painter’s tape
• Permanent marker
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs - Qty: 6
• Upper intake manifold gasket set - Qty: 1
• Throttle body gasket - Qty: 1
• Dielectric grease - Qty: 1 small tube
• Mass airflow sensor-safe intake cleaner - Qty: 1 can

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📋 Before You Begin

• Park your Highlander on level ground, shift to Park, and set the parking brake.
• Let the engine cool completely before starting.
• Use a 10mm socket to disconnect the negative battery cable and move it aside so it cannot spring back to the terminal.
• Use painter’s tape and a permanent marker to label hoses and electrical connectors before unplugging them. This makes reassembly much easier.
• Blow dirt away from the top of the engine using a compressed air nozzle before removing parts.
• Plan to replace all six spark plugs together. Do not replace only one bank.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Engine Cover

• Put on safety glasses and nitrile gloves.
• Lift the plastic engine cover straight upward by hand to release the rubber mounting grommets.
• Set the cover aside in a safe spot.
• Tip: Pull evenly, not sideways.

Step 2: Disconnect the Battery

• Use a 10mm socket and ratchet to loosen the negative battery terminal nut.
• Remove the negative cable from the battery post.
• Wrap the cable end with a clean shop towel so it cannot touch the battery.

Step 3: Remove the Air Intake Tube

• Use a flathead screwdriver to loosen the hose clamp at the throttle body.
• Use a flathead screwdriver to loosen the hose clamp at the air filter box.
• Use needle-nose pliers to release any small hose clamps connected to the intake tube.
• Unplug the mass airflow sensor connector by pressing the tab and pulling straight back.
• Remove the intake tube and set it aside.

Step 4: Remove Throttle Body From the Intake Manifold

• Use painter’s tape and a permanent marker to label the throttle body connector.
• Unplug the throttle body electrical connector by pressing the lock tab.
• Use a 10mm socket and ratchet to remove the throttle body mounting bolts.
• Move the throttle body aside without forcing the coolant hoses.
• Remove and discard the old throttle body gasket.
• Torque to 10 Nm (89 in-lbs) during reassembly.

Step 5: Label and Disconnect Intake Manifold Hoses and Connectors

• Use painter’s tape and a permanent marker to label each hose and connector on the upper intake manifold.
• Use needle-nose pliers to release spring clamps on vacuum hoses.
• Pull hoses off gently by twisting them first, then pulling straight off.
• Use a trim clip removal tool to release harness clips from the intake manifold brackets.
• Tip: Take phone photos before unplugging.

Step 6: Remove Upper Intake Manifold Brackets

• Use a 12mm socket, extension, and ratchet to remove the rear and side intake support bracket bolts.
• Use a magnetic pickup tool if a bolt is hard to reach.
• Set the bolts aside in order on a clean towel.

Step 7: Remove the Upper Intake Manifold

• Use a 12mm socket, 6-inch extension, and ratchet to remove the upper intake manifold bolts and nuts.
• Lift the upper intake manifold straight up and away from the engine.
• Remove the old intake manifold gaskets.
• Immediately cover the exposed intake ports with clean shop towels.
• Torque to 21 Nm (15 ft-lbs) during reassembly.

Step 8: Clean the Work Area

• Use a shop vacuum to remove loose dirt around the ignition coils.
• Use a compressed air nozzle to blow dirt away from the spark plug wells.
• Do not blow debris into uncovered intake ports.
• Tip: Clean now to protect the engine.

Step 9: Remove the Front Ignition Coils

• Use a 10mm socket and ratchet to remove the ignition coil hold-down bolt from each front coil.
• Unplug each ignition coil connector by pressing the release tab.
• Pull each coil straight upward with a slight twist.
• An ignition coil is the part that sends high voltage to the spark plug.
• Torque to 10 Nm (89 in-lbs) during reassembly.

Step 10: Remove the Front Spark Plugs

• Use a 5/8-inch spark plug socket, 6-inch extension, and ratchet to loosen each front spark plug.
• Turn counterclockwise slowly until each plug is fully loose.
• Lift each plug out using the rubber insert inside the spark plug socket.
• Check each old plug for oil, fuel smell, heavy carbon, or broken porcelain.

Step 11: Install the Front Spark Plugs

• Place one new iridium spark plug into the 5/8-inch spark plug socket.
• Start the plug by hand using the extension only, with no ratchet attached.
• Turn clockwise several turns until it threads smoothly.
• Use a 3/8-inch drive torque wrench and 5/8-inch spark plug socket to tighten each spark plug.
• Torque to 18 Nm (13 ft-lbs)
• Tip: If it resists, stop immediately.

Step 12: Reinstall the Front Ignition Coils

• Apply a small dab of dielectric grease inside each ignition coil boot. Dielectric grease helps seal moisture out of electrical connections.
• Push each coil straight down onto its spark plug.
• Use a 10mm socket and torque wrench to tighten each coil bolt.
• Torque to 10 Nm (89 in-lbs)
• Reconnect each ignition coil electrical connector until it clicks.

Step 13: Remove the Rear Ignition Coils

• With the upper intake manifold removed, use a 10mm socket, 3-inch extension, and ratchet to remove the rear ignition coil hold-down bolts.
• Unplug each rear ignition coil connector by pressing the release tab.
• Pull each rear coil straight upward with a slight twist.
• Use a universal joint socket adapter if access is tight.

Step 14: Remove the Rear Spark Plugs

• Use a 5/8-inch spark plug socket, 6-inch extension, universal joint socket adapter, and ratchet to loosen each rear spark plug.
• Turn counterclockwise slowly until the spark plug is loose.
• Lift each spark plug out carefully using the spark plug socket.
• Do not drop dirt or hardware into the spark plug wells.

Step 15: Install the Rear Spark Plugs

• Place one new iridium spark plug into the 5/8-inch spark plug socket.
• Start the spark plug by hand using only the extension.
• Use a 3/8-inch drive torque wrench, extension, and 5/8-inch spark plug socket to tighten each rear spark plug.
• Torque to 18 Nm (13 ft-lbs)
• Repeat for all three rear plugs.

Step 16: Reinstall the Rear Ignition Coils

• Apply a small dab of dielectric grease inside each rear ignition coil boot.
• Push each coil straight down until fully seated.
• Use a 10mm socket and torque wrench to tighten the coil bolts.
• Torque to 10 Nm (89 in-lbs)
• Reconnect each ignition coil connector until it clicks.

Step 17: Reinstall the Upper Intake Manifold

• Remove the shop towels from the intake ports.
• Use a shop vacuum around the ports before installing the manifold.
• Install the new upper intake manifold gaskets into position.
• Set the upper intake manifold straight down onto the engine.
• Start all bolts and nuts by hand first.
• Use a 12mm socket and torque wrench to tighten the manifold fasteners evenly from the center outward.
• Torque to 21 Nm (15 ft-lbs)

Step 18: Reinstall Intake Brackets, Hoses, and Connectors

• Use a 12mm socket and ratchet to reinstall the intake support brackets.
• Reconnect all labeled vacuum hoses by hand.
• Use needle-nose pliers to reposition spring clamps.
• Use a trim clip removal tool to guide harness clips back into their brackets.
• Reconnect all electrical connectors until they click.

Step 19: Reinstall the Throttle Body

• Install the new throttle body gasket.
• Set the throttle body back onto the intake manifold.
• Use a 10mm socket to start all throttle body bolts by hand.
• Use a 10mm socket and torque wrench to tighten the bolts evenly.
• Torque to 10 Nm (89 in-lbs)
• Reconnect the throttle body electrical connector until it clicks.

Step 20: Reinstall the Air Intake Tube

• Install the air intake tube onto the air filter box and throttle body.
• Use a flathead screwdriver to tighten both hose clamps snugly.
• Reconnect any small hoses using needle-nose pliers for spring clamps.
• Reconnect the mass airflow sensor connector until it clicks.

Step 21: Reconnect the Battery and Reinstall the Engine Cover

• Use a 10mm socket and ratchet to reconnect the negative battery cable.
• Make sure the terminal is fully seated and snug.
• Push the plastic engine cover down by hand until the grommets seat.

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✅ After Repair

• Start the engine and let it idle for several minutes.
• The idle may be slightly high or uneven at first while the throttle relearns after the battery disconnect.
• Listen for hissing noises, which may indicate a vacuum leak from a loose hose or intake gasket.
• Check that the Check Engine light stays off.
• Take a short test drive, then recheck that the intake tube clamps and connectors are secure.
• If the engine runs rough, shut it off and recheck ignition coil connectors, vacuum hoses, and intake manifold seating.

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💰 DIY vs Shop Cost

Shop Cost: $650-$1,000 (parts + labor)

DIY Cost: $90-$180 (parts only)

You Save: $470-$820 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 3-4 hours.

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